Magnetic head core of large grain size and low porosity

ABSTRACT

1. A MAGNETIC HEAD CORE CONSISTING ESSENTIALLY OF SINTERED FERRITE CONSISTING OF A PLURALITY OF GRAINS, HAVING AN AVERAGE GRAIN SIZE GREATER THAN 30 U AND A POROSITY LESS THAN 0.1% BY VOLUME, AND HAVING A COMPOSITION CONSISTING OF NI-ZN FERRITE, MN-ZN FERRITE, LI-ZN FERRITE, CU-ZN FERRITE, MG-ZN FERRITE OR Y-IRON GARNET.

TEM PERATU F? E Oct. 22, 1974 HIRQYUKI cHlBA ET AL I 35843541 MAGNETICHEAD coma: OF LARGE (mam SIZE AND LOW POROSITY Original Fild Feb. 9,1967 2 Sheets-Sheet 1 \I/IOOO- LL LL. M I

GRA|N SIZE (a) x TIME PERIOD I3 FIG-4 12 FIG.7

0a. 22,1914 HI UK, CHIBA ETAL 3,043,541

MAGNETIC HEAD CORE 0! LARGE GRAIN SIZE AND LOW POROSITY Original FiledFeb. 9. 1967 2 Sheets-Sheet 2 HEAD OF THE INVENTION RELATIVE MAXIMUMOUTPUT VOLTAGEI v) I I l I I 0.4 0.6 0.8IO 2 4 FREQUENCY (MC) FIG. I I

United States Patent 3,843,541 MAGNETIC HEAD CORE 0F LARGE GRAIN SIZEAND LOW POROSITY Hiroyuki Chiba, Neyagawa, Yoshio Tawara, Kadoma, andEiichi Hirota, Sakai, Japan, assignors to Matsushita Electric Industn'aiCo., Ltd., Osaka, Japan Continuation of abandoned application Ser. No.29,355, Apr. 20, 1970, which is a division of application Ser. No.614,889, Feb. 9, 1967, now Patent No. 3,557,266, dated Jan. 19, 1971.This application Mar. 14, 1973, Ser. No. 341,491

Int. Cl. C04b 35/26, 35/30, 35/38 US. Cl. 252-62.57 3 Claims ABSTRACT OFTHE DISCLOSURE An improved magnetic head core is provided. The magnetichead core consists essentially of sintered ferrite having an averagegrain size greater than 30,0. and a porosity of less than 0.1% byvolume.

This is a continuation, of application Ser. No. 29,355, filed Apr. 2,1970, now abandoned, which is a Rule 147 divisional application of Ser.No. 614,889, filed Feb. 9, 1967, now US. Pat. No. 3,557,266.

This invention relates to a method for making a magnetic head core for avideo tape recorder and more particularly to a method for making amagnetic head core comprising hot-pressed ferrite.

A magnetic head for a video tape recorder has heretofore had a veryshort life and has had to be exchanged after every 100 to 200 hours ofoperation. In addition, the image reproduced by a video tape recorderhas been inferior in sound-to-noise ratio (8/ N) to the input imageproduced by a TV camera. The inferiority is mainly due to the magnetichead and to the tape. Therefore, many efforts have been aimed at theimprovement of the magnetic head material and structure for obtaining anentirely satisfactory image reproduced by a video tape recorder.

A magnetic alloy such as Fe-Al or Fe-Al-Si having high permeability hasbeen widely used heretofore as a magnetic head pole tip but has not'been entirely satisfactory in the electro-magnetic properties of highfrequency and in the operational life because it does not have a highelectrical resistivity and a high mechanical hardness.

Ferrite has a high magnetic permeability and a high electricalresistivity and is in a high potential for use in a magnetic head core.Various attempts have been directed to the preparation of a magnetichead core comprising a ferrite in a sintered form or in a single crystalform. The ferrite in a sintered form is usually prepared by a so-calledsintering method comprising a heating process without pressure or by aso-called hot-pressing method comprising a hot-pressing process. Theferrite prepared by the sintering process is apt to have a highporosity. The hot-pressing process makes a ferrite having a smallaverage grain size and a low porosity. Therefore, it has been difficultto control both the porosity and the average grain size of ferrite byemploying, as a single operation, either the sintering process or thehot-pressing process. The magnetic head core requires a head gap of anorder of about 1 and a high mechanical strength which results in a longoperational life of the magnetic head. This entails controlling both theaverage grain size and the porosity of the sintered ferrite.

A ferrite in a single crystal form has a high potential for use in amagnetic head core because it is of no porosity and because of itssingle grain form. However, it is difficult to manufacture, in a highproduction yield and at a low cost, the single crystal of ferrite havinga Patented Oct. 22, 1974 desired composition distributed uniformlythroughout the crystal and being of a size sufficiently large for makinga magnetic head. A single crystal of ferrite is usually hard but isbrittle. The brittleness prevents the single crystal of ferrite frombeing easily shaped to a magnetic head pole tip by simple machine work,and also causes damage due to high running speed of the tape.

A magnetic head core according to a conventional method is formed byadhering a pole tip made of a single crystal ferrite or aforesaidmagnetic alloy to a yoke comprising a sintered ferrite. Such a formrequires additional complicated processes.

An object of the invention is to provide a method for making a magnetichead core having improved electrical and magnetic properties at highfrequencies.

-.A further object of the invention is to provide a method for making amagnetic head core having a long operational life. 5 1

Another object of the invention is to provide a method for making amagnetic head core with a pole tip and a yoke in a single body.

These and other objects will be apparent upon consideration of thefollowing description taken together with the accompanying drawingswherein:

FIG. 1 is a perspective view of a magnetic head core comprising twohot-pressed ferrite elements according to the invention;

FIG. 2 is a front view of the magnetic head core illustrated in FIG. 1;

FIG. 3 is a cross-sectional view of the magnetic head core illustratedin FIG. 1, which is taken along the opposed abutting surfaces of theelements, i.e. along line 3-3;

FIG. 4 is is a graphic representation of the heating schedule for makinga sintered ferrite in accordance with the invention;

FIG. 5 is a graph illustrating the operation life versus the averagegrain size of said sintered ferrite;

FIG. 6 is a perspective view of a shaped bar comprising a hot-pressedferrite according to the invention;

FIG. 7 is a cross-sectional view of a shaped bar illus trated in FIG. 6;4

FIG. 8 is a perspective view of a joined bar which is provided withglass fibers. 1

FIG. 9 is a perspective view of an adhered bar which is polished at thepole tip surface.

FIG. 10 is a perspective view of a grooved bar.

FIG. 11 is a graph illustrating the freqeuncy versus the relativemaximum output voltage of the magnetic head prepared by a methodaccording to the invention with reference to the frequencycharacteristics of a conventional magnetic head core comprising amagnetic alloy of Sendust.

Before proceeding with a detailed description of the novel method formaking a magnetic head core comprising a hot-pressed ferrite accordingto the invention, the construction of the magnetic head core will beexplained with reference to FIGS. 1, 2 and 3 of the drawings whereinreference character 10 desingates, as a whole, a magnetic head corecomprising two elements 1 and 2 of hot-pressed body of ferrite. The twoelements 1 and 2 comprise yokes 1b and 2b in a thickness of 0.3 to 1.0mm. and are joined together by using glass spacers 3 and 4. Pole tips 1aand 2a have a thickness approximately equal to the track width ofconventional tape. The width of said glass spacers 3 is from 0.1 to 1.0',u. A notch 5 is used for holding a glass fiber which is infiltratedinto a gap between the two elements 1 and 2 during the heating processfor melting the glass.

The novel method for making the magnetic head core 10 according to theinvention can be accomplished in four steps, i.e. (1) preparation of aprocess hot-pressed body of ferrite composition, (2) shaping of saidhot-pressed body into a bar having specified configuration, (3) joiningtwo of said bars, and (4) a cutting process for slicing said joined barinto magnetic head cores having a desirable thickness and a specifiedcross-sectional shape, as set forth hereinafter.

The preparation process comprises heating initially a cold-pressed bodyof a ferrite composition in a die up to a temperature higher than 1000C., holding said coldpressed body at the said temperature for a timeperiod longer than 30 minutes while pressing said cold-pressed body fora time period longer than 10 minutes so as to form a sintered bodyhaving a high density, heating said sintered body up to a temperaturehigher than the initial heating temperature without pressure, holdingsaid sintered body at the higher temperature for a time period longerthan 30 minutes while pressing said sintered body for a time periodlonger than 10 minutes so as to increases the grain size of saidsintered body and then furnace-cooling said sintered body to roomtemperature (about 25 to about 35 C.) without pressure.

In the shaping process, said resulting sintered body is cut into a barhaving a cross-sectional shape defined by FIG. 7. The tip face ispolished initially with a lapping agent such as Cr O and then finallywith a diamond paste.

The top part of said bar is partially coated with a titanium evaporationfilm of a thickness of 0.1 to 1.0 t. Two thus-produced bars, one ofwhich is not provided With the said evaporation film, are joined at thepolished surfaces by using a holding tool. Said titanium evaporationfilm keeps easily the tip gap at 0.1 to 1.0 t, when the two bars arejoined. A glass fiber of 0.1 to 0.4 mm. diameter is placed on the hollow5 and in the interior of the tip gap (cf. FIG. 8). The joined bars withglass fibers are heated in a non-oxidizing atmosphere such as N gas at atemperatures which makes it possible for the glass fibers to melt and topenetrate into the tip gap and into the bottom joint space, and are thenfurnace-cooled to room temperature so as to form an adhered head corehaving a tip gap of 0.1 to 1.0 ,u. The adhered bar is lapped andpolished at the top surface of tip to remove the said evaporatedtitanium film therefrom.

The resultant adhered bar is cut into a magnetic head core having theaforesaid thickness and the crosss-sectional shape defined by FIGS. 1 to3 by using a conventional and suitable cutting tool, such as a diamondwheel.

Any ferrite composition having a high magnetic permeability and a highmaximum magnetic induction Bm is operable. Operatable ferritecompositions are Ni-Zn ferrite, Mn-Zn ferrite, Li-Zn ferrite, Cu-Znferrite, Mg-Zn ferrite, and Y-iron garnet (Y Fe O Preferable ferrite,compositions are listed in Table 1 wherein compositions are shown inmole percent:

It has been discovered according to the invention that a magnetic headcore of superior high frequency electrical properties and of superioroperational life can be prepared by employing a ferrite having both aporosity less than 0.1% by volume and an average grain size higher than30 51,.

It has been difficult to prepare the ferrite having both a porosity lessthan 0.1% by volume and an average grain size higher than 30' i1. byemploying a single hot-pressing operation. A single hot-pressingoperation at a low temperature of 800 C. to 1250 C. results in a highdensity 4 but in a small average grain size. A single hot-pressingoperation at a high temperature of 1250 C. to 1400 C. results in a largeaverage grain size but in a low density which is due to a difliculty inremoving gas previously included in the cold-pressed body.

It has been discovered according to the invention that a ferrite havingboth a porosity less than 0.1% by volume and an average grain sizehigher than 30 ,u. can be obtained by employing a combination ofhot-pressing and heating without pressure.

Referring to FIG. 4, a dotted line indicates a time period at which apressure is applied to the ferrite composition. A cold-pressed body of agiven ferrite composition is heated up to a temperature A of 1100 to1250 C. and is held at this temperature for a time period 2 of 30minutes to 5 hours so as to form a sintered body of ferrite. During saidtime period g, the ferrite composition is hotpressed at a pressurehigher than 50 kg./cm. (kilograms per square centimeter) for a timeperiod of 10 minutes to 2 hours. The time period of hot-pressing can bepositioned at any desirable time of the time period 3. A preferable timeperiod of hot-pressing is positioned at the final step of the timeperiod g as shown in FIG. 4 because gas such as air included in thecold-pressed body easily comes out during heating without pressure.

When the cold-pressed body is treated with only hotpr-essing during thetime period g, the sintered body of ferrite is apt to comprise a phaseof a-Fe O which impairs the magnetic properties of the resultantsintered body of ferrite. The temperature A is a temperature at whichthe sintered ferrite does not show a remarkable grain growth even whenheated for a long period higher than 5 hours. Therefore, the sinteredbody obtained at the time period g has a high density due to thehot-pressing operation but has a small grain size less than 5 a.

The so-produced sintered body is subsequently heated up to a temperature3 of 1250 to 1400 C. and is held at this temperature for a time period 1of 30 minutes to 10 hours. During the period X, the sintered body ispressed at a pressure higher than 50 kg./cm. for a time period of 10minutes to 3 hours. The hot-pressing can be applied at any desirabletime and is applied preferably at the final period of the time period 1.The heat-treatment at the time period 1 is eifective for obtaining alarge grain size higher than 30 1..

Following the time period X, the sintered body is furnace-cooled withoutpressure to room temperature.

FIG. 5 wherein operational life is defined by a time period in hours,during which an output voltage of a magnetic head is lowered by 3 dB,makes clear that a ferrite having an average grain size higher than 30p.can insure an operational life longer than 500 hours. Preferable averagegrain size is a size higher than 50,44. The average grain size isdetermined by calculating a number of grains at a given area ofmicroscopic photograph in a given magnification in association with amethod defined at pages 1690 to 1693 of the 1955 Book of ASTM, i.e. B89-52.

A mixture of a ferrite composition mentioned above is calcined at atemperature of 800 to 1100 C. and is pressed, with or without any bindermaterial, at room temperature to any desired shape in a per se wellknown ceramic fabrication technique. A cold-pressed body so produced isplaced in a die made of any refractory material while being embeddedwith a powder of refractory material and is heated in a vertical typeelectric furnace. An upper punch can supply a pressure to thecold-pressed body through the embedding powder by using a conventionalpressing machine.

It is important that the embedding powder does not react with theferrite composition and does not adhere to the ferrite body during aheat-treatment. It has been discovered according to the invention thatan operable powder is of A1 which is made of molten A1 0 having a purityhigher than 99% by weight and has an average grain size of 60 to 300Operable materials for a die and a punch are any refractory materialhaving a high mechanical strength at the high temperature ofhot-pressing and a high resistance to oxidation and to reaction with theAl O Preferable materials are silicon carbide and silicon nitride. Aconventional graphite is not suitable for hotpressing ferritecomposition because the graphite produces carbon monoxide which easilyreduces the ferrite compositions.

The novel heat-treatment for making a hot-pressed body of ferritecomposition according to the invention can be carried out in air. Apreferable heating atmosphere is a reduced pressure of air lower than100 mm. Hg. The reduced pressure tends to make the heating apparatuscomplicated but has a great effect to reduce the porosity of theresultant sintered body of ferrite composition.

The resultant sintered body is cut to a bar having for example, a 2.3mm. thickness, a 5 mm. Width and a 35 mm. length. The bar is lapped by aconventional lapping agent such as Cr O and is shaped to a bar having acrosssectional view defined by FIG. 7 in a per se well known technique.After shaping, the bar is polished at surfaces 11 and 12 of twoprojected parts by a diamond polishing paste. Two of the so-producedbars are joined together at the surfaces 11 and 12 by using a per seconventional holding tool. One of the two is partially coated at thesurface 12 with a vacuum evaporation metal film 13 of a 0.1 to 1.0thickness and about 150a width in a per se conventional manner. Thecoated film assures a head-gap of 0.1 to 1.0,u. when the two bars arejoined. The evaporation film can be made of any suitable metal, such astitanium.

Referring to FIG. 8, the joined bar a! is placed in a furnace in such away that the hollow or notch 5 is positioned upward. Two glass fibers 15and 16 are put on the notch 5 and at the interior of the tip-gap. Thejoined bar 29 provided with the glass fibers 15 and 16 is heated in anon-oxidizing atmosphere, such as nitrogen, up to a temperature at whichthe glass fibers 1S and 16 melt and infiltrate into the gap 17 betweenthe two bars, and are then furnace-cooled to room temperature. Anonoxidizing atmosphere is important for preventing the segregation ofu'Fe O segregates in the sintered ferrite body when the sintered body offerrite is heated in air at a temperature of 500 C. to 900 C. whilecontacting with the molten glass. The segregated ot-Fe O impairs themagnetic properties of the ferrite. It has been discovered according tothe invention that the non-oxidizing atmosphere during melting the glassfiber has a great effect to suppress the undesired segregation of a-Fe OA glass fiber composition also has a great effect on the operationallife of the resultant magnetic head. Necessary properties of the glasscomposition are a mechanical hardness approximately equal to that of thesintered ferrite, an excellent adhesion between the two bars, and amelting point below 900 C.

An operable glass composition is a composition having a mechanicalhardness in a Vickers scale diifering from that of a ferrite by lessthan :30% A superior magnetic head core can be prepared by employing aglass composition having a Vickers hardness differing from that of theferrite by less than :15 The ferrite compositions listed in Table 1 canform an excellent magnetic head core when combined with the glasscompositions listed in Table 2 wherein compositions are expressed inweight percent:

TABLE 2 Ingredients and properties:

Slog

Softening temperature O.) L near expansion coefficient (X 10 Vlckershardness The Mn-Zn ferrite and Ni-Zn ferrite according to the inventionhave approximately, as an average value, a linear thermal expansioncoefficient of 90- 10 and 96 l0- and a Vickers hardness of 600- and 630,re spectively. The glass compositions listed in Table 2 have a Vickershardness dilfering by less than 130% from those of the Mn-Zn ferrite andNi-Zn ferrite and can make a magnetic head core having a superioroperational life. Thermal expansion coefficient of the glass compositiondoes not have a critical effect on the operational life.

For example, a glass composition consisting of 2.6 wt. percent of SiOwt. percent of PbO, 1.0 wt. percent of A1 0 8.8 wt. percent of ZnO and7.6 wt. percent of B 0 has a linear thermal expansion coefficient of 95Xl0 which is close to those of the above ferrites and a Vic-kers hardnessof 300 which is greatly lower than those of the above ferrites andcannot make a magnetic head core having a superior operational life.

The adhered bars 20 are lapped and polished at the surface 18 of the tipfor removing the part having the vacuum evaporation film and for forminga curvature at the surface as shown in FIG. 9, by a per se well knownmethod. The polished bar of FIG. 9 is grooved at the polished tipsurface 18 as shown in FIG. 10, by using a diamond wheel cutter. Eachgroove 21 has a width of, for example, about 0.6 mm. and a depth of, forexample, about 0.5 mm. Each hill 22 between the grooves has a width ofabout 0.35 mm. which is approximately equal to the sum of track width ofa tape and width of the diamond wheel which is used for slicing in a wayset forth hereinafter. An important feature is that the grooves areprepared in a way that an angle 0 between a side wall 24 of the grooveand the bottom surface 23 is essentially The grooved bar is sliced intoa magnetic head core having a thickness of about 1 mm. by a diamondwheel cutter in a per se conventional slicing method. The so producedmagnetic head core is polished at a surface shown by the arrow in FIG.3.

Such a shape of polo tip as defined by FIG. 3 in accordance with theinvention has an advantage that the thickness of the pole tip 1a and 2adoes not vary with decrease in the height of the pole tip duringoperation. In addition the novel shape makes it possible to increase themechanical strength of the yoke 1b and 2b and to reduce the reluctanceof the yoke 1b and 2b. Another advantage is that the magnetic head corecan be easily prepared by a simple machine operation, which results in alow cost.

A magnetic head core prepared by the novel hot-pressing method accordingto the invention can form a superior magnetic head which has an optimumrecording current less than half of that of a conventional magnetic headcomprising, for example, a core made of a magnetic alloy, such asSendust.

The following are non-limitative illustrative examples of the presentlypreferred embodiments of the invention:

EXAMPLE 1 A mixture of 32 mole percent of MnO, 16 mole percent ZnO and52 mole percent Fe O is calcined at 1100 C. for 2 hours, ground into afinely divided powder and pressed into a disk of 36 mm. in diameter and5 mm. in height at a pressure of 300 kg./cm. The pressed body isinserted in a die made of silicon nitride while being embedded in meltedalumina powder having an average particle size of 80,11. and is thenheated in an electrical furnace provided with a pressing machine to 1200C. within 1 hour in air. The pressed body is held at this temperaturefor 1 hour while being hot-pressed by a pressure of 250 kg./cm. for 30minutes at a final period of the 1200 C. heating period so as to form asintered ferrite. The sintered ferrite is subsequently heated to 1350 C.without pressure within minutes and is held at this temperature for 4hours while being hot-pressed by a pressure of 250 kg./cm. for the finalminutes of the 1350 C. heating period.

Finally, the sintered body is furnace-cooled to room temperature withoutpressure. The resultant sintered body having a porosity less than 0.1%by volume and an average grain size of about 200 p. is shaped, in amanner similar to that of the above description, to a bar having across-sectional view according to FIG. 7.

One of the so-produced bars is coated with a titanium evaporation filmin a manner similar to the above description and is joined to anothersuch bar having no evaporation film by using a per se conventional tool.Two glass fibers of a composition consisting of 22.5 mole percent of SiO21 mole percent of Na O and 56.5 mole percent of B 0 are attached to thejoined bars in a similar way to that of the above description. Thejoined bars provided with two glass fibers are heated in nitrogen gas at750 C. for 10 minutes and then furnace-cooled to room temperature.

The adhered bars are sliced into a magnetic head core in a mannersimilar to that of the above description after being grooved andpolished. A magnetic head comprising the so-produced magnetic head corehas a maximum reproducing output voltage of 3.5 mv. at a frequency of 4Me. and at a tape speed of 20 meters/second. FIG. 11 shows that thenovel magnetic head core is superior in frequency vs. relative maximumoutput voltage to the conventional magnetic head core comprising aconventional magnetic alloy such as Sendust consisting of Fe-Al-Si. Themagnetic head core so produced has a superior operating life such thatthe initial maximum output voltage of 3.5 mv. does not change withinhours of operation at a frequency of 4 Me. and at a tape speed rate of20 meters/second and decreases only by less than 0.5 mv. after 1000hours of operation.

EXAMPLE 2 A mixture of 50 mole percent of Fe O 35 mole percent of NiOand 15 mole percent of ZnO is fabricated into a resultant sintered bodyin a similar way to that of Example 1 except for the initial heatingtemperature. In this Example 2, the initial heating temperature is 1250C. The resultant sintered body has a porosity less than 0.1% by volumeand an average grain size of 100 ,u.

The fabrication for a magnetic head core is also effected in a similarway to that of Example 1 except for the glass composition. Glass fibersconsisting of a composition defined by Example B in Table 2 areinfiltrated into the head gap by heating in nitrogen gas at 900 C. for10 minutes.

A magnetic head comprising the so-produced magnetic head core has amaximum reproducing output voltage at 4.0 mv. at a frequency of 2.7 Mo.and at a tape speed of 12 meters/second. The initial maximum reproducingout-put voltage of 4.0 mv. decreases only by less than 0.2 mv. after1000 hours of operation.

Having thus disclosed the invention, what is claimed is:

1. A magnetic head core consisting essentially of sintered ferriteconsisting of a plurality of grains, having an average grain sizegreater than 30 n and a porosity less than 0.1% by volume, and having acomposition consisting of Ni-Zn ferrite, Mn-Zn ferrite, Li-Zn ferrite,Cu-Zn ferrite, Mg-Zn ferrite or Y-iron garnet.

2. A magnetic head core as claimed in claim 1 wherein the average grainsize is 100 n.

3. A magnetic head core as claimed in claim 1 wherein the average grainsize is 200 ,u.

References Cited UNITED STATES PATENTS 3,472,780 10/1969 Stuijts252-62.62

FOREIGN PATENTS 1,052,455 12/1966 Great Britain 252--62.62

1,094,995 7/1962 Germany 252-6256 JACK COOPER, Primary Examiner U.S. Cl.X.R.

1. A MAGNETIC HEAD CORE CONSISTING ESSENTIALLY OF SINTERED FERRITE CONSISTING OF A PLURALITY OF GRAINS, HAVING AN AVERAGE GRAIN SIZE GREATER THAN 30 U AND A POROSITY LESS THAN 0.1% BY VOLUME, AND HAVING A COMPOSITION CONSISTING OF NI-ZN FERRITE, MN-ZN FERRITE, LI-ZN FERRITE, CU-ZN FERRITE, MG-ZN FERRITE OR Y-IRON GARNET. 